There are many applications in which the existing characterization or stress on a material or a portion of that material needs to be measured accurately and reliably. When stressed, the inter-atomic spacing of the lattice atoms of the material is displaced. Other conditions can alter the microstructure of the material as well, such as heat treatment or fatigue, for example. Past approaches to measure the stress state existing within a material have used very complex and expensive equipment that is difficult or impossible to move to a field site.
One stress measurement approach uses radiation. For example, an x-ray scattering measurement technique, called Laue Backscatter Diffraction, determines a change in lattice spacing based on the statistical scattering properties of the material. But Laue Backscatter Diffraction requires a radiation wavelength equal to the atomic spacing and specific angles to correspond exactly to the spacing of the atoms to create a sharp image of the coherence of the lattice. A similar technique utilizes a beam of Low Energy Electron Diffraction (LEED) images to acquire the same type of data. For electrons of energy ˜20 eV (electron volts), the wavelength is about the same as for x-ray photons of ˜20 keV (thousand electron volts) or about 2.5 Angstroms. Other stress measurement techniques based on radiation include neutron and synchrotron radiation. All radiation approaches are complex, require laboratory set-ups such as vacuum systems, radiation approval, complex shielding, and are very expensive and difficult to move out of the lab. Data interpretation is also complex and localized. Moving the measurement location also often produces different results.
Other stress measurement approaches are destructive, e.g., material is removed from the structure by drilling a hole. Destroying part of the material may create a significant flaw site and is generally not desired in critical structures. One destructive method mounts a strain gauge on the measurement site. At the center of the gauge, a small hole is drilled that relaxes the stress at and around the hole site. The strain gauge measures the small deformations in the remaining material that surround the hole. Another approach uses laser holography to examine the deformation at the site around the hole.
Destructive mechanical microanalysis of a structure or a part is often used to examine grain micro-structure and alloy complexity with optical, electron, and x-ray analysis tools. These tools are all destructive in nature and can provide satisfactory assessment of a part undergoing destructive testing, but not of the actual part or material in its working environment.
Ultrasonic stress measurement approaches are based on changes in the ultrasonic propagation velocity with stress and microstructure. Early work on the measurement of bolt tension was based on the pulse-echo time-of-flight changes that occur when a fastener is placed under load. But these measurements require that the unloaded initial conditions of the bolt be measured before the bolt is stressed. As the bolt is loaded, it elongates and the velocity of sound propagating through the bolt changes. Both of these effects contribute to the altered time measured by the ultrasonic instrument. See for example U.S. Pat. No. 4,062,227.
Another stress measurement approach is based on a phase shift locking instrument called a pulsed phase locked loop as described for example in U.S. Pat. Nos. 4,363,242 and 4,624,142. The change in the frequency of the signal source locked to a specific phase point in a bolt or other object is determined. (A change in frequency is related to a change in velocity and length of the acoustic propagation). As the bolt elongates, the frequency drops keeping the phase of the system locked at a relative phase difference of ninety degrees (referred to as “quadrature”). Although the pulsed phase locked loop approach has a higher resolution than that achieved using the technique described in U.S. Pat. No. 4,062,227, it still requires an initial measurement of the unloaded material. Other ultrasonic approaches use combinations of compression, shear, and special polarization waves to determine changing stress states in a material. But they are more complex, requiring measurements along different paths, orientations, and/or polarizations.
For ferromagnetic materials, one can measure the changes in ultrasonic propagation during magnetic saturation. This approach, developed by Namkung and Heyman and described in “Residual Stress Characterization with an Ultrasonic/Magnetic Technique,” Nondestructive Testing Communications, Vol. 5, September 1984, shows that the existing state of stress can be determined from the velocity change derivatives with respect to external magnetization. Barkhausen Noise is another derivative approach based on listening to the acoustic emissions that occur when ferromagnetic materials are magnetized. Depending on the initial state of stress, the emitted noise is altered during magnetization and gives some indication of the initial state of stress. This approach has been difficult to use quantitatively.
All of these ultrasonic stress and microstructure measurements are compromised by changes in thermal conditions because the speed of sound is strongly influenced by temperature. Therefore, the temperature of the material being analyzed must be determined, and if the temperature of the material is not uniform, the measurement is further complicated.
Given the various shortcomings and/or complexities of the stress and microstructure measurement approaches described above, a stress and microstructure measurement device is needed that is relatively simple, inexpensive, robust, preferably portable, and does not require that an unloaded initial conditions of the material be measured or otherwise known in order to determine a characteristic of the material like stress.